During the manufacture of products made of nonwoven fibers, it is typically necessary to bond the nonwoven fibers through the application of heat. After the fibers have been bonded, they are typically used thereafter to form many types of products including, for example, personal care products such as sanitary napkins, incontinence pads, diapers, absorbent bed pads, and the like. The process of heating the nonwoven fibers is typically performed with a convection oven as an early step in a continuous manufacturing process that begins with the bonding of the nonwoven fibers and ends with the production of a final product formed from the bonded fibers. The continuous manufacturing process typically involves multiple machines which operate sequentially on a single continuously moving web of nonwoven fibers.
The convection oven used for bonding the continuously moving web of nonwoven fibers typically includes a conveyor mechanism for continuously carrying the nonwoven fiber web through the interior of the oven. As the web moves through the oven, the speed of the conveyor and oven temperature are such that the web is exposed to the appropriate amount of heat necessary for bonding as the web travels through the interior length of the oven. If, for any reason, the temperature inside the oven is too low as the web travels through the oven, the web will be exposed to insufficient heat and will not be properly bonded. In addition, if, for any reason, the web were to stop for any length of time within the oven, the web may be overexposed to heat resulting in overbonding, overdrying and/or burning. Product that is not processed to specifications, e.g., overbonded or underbonded, can affect the efficiency of downstream processes. For example, an overbonded core in a sanitary napkin manufacturing line may be too stiff to fold, and an underbonded absorbent core may be too bulky or weak to handle. These problems increase waste levels and decrease manufacturing efficiencies.
During the continuous manufacturing process described above, machines in the manufacturing line other than the oven used for bonding the web may require stoppage of the manufacturing line. When such a stoppage occurs, the portion of the web residing inside the convection oven will also stop. In order to avoid any overprocessing of the web material that has stopped inside the oven, the flow of heat inside the oven directed onto the web must either cease or be diverted away from the web when the manufacturing line stops. However, in order to ensure that, upon restarting of the manufacturing line, the portion of the web exiting the oven will be sufficiently bonded, the oven must be maintained in a hot state such that little or no time transpires between the time the manufacturing line is switched back on and the time the oven reaches its appropriate operating temperature.
Several bypassing systems have been proposed for diverting the flow of heat inside an oven away from a continuous product line. Two such systems are shown in U.S. Pat. No. 4,590,916 by Konig and U.K. Patent Application No. GB 2234421A by Norfolk, both of which are directed to baking ovens. In these systems, the ovens may operate in either a running mode or a bypass mode. During the running mode, hot air circulates through a cooking zone in the oven containing food items thereby impinging on the items being baked. In the bypass mode, hot air continues to circulate in the oven, however, the recirculation path is such that the air flow within the oven is diverted around the cooking zone.
The prior art systems identified above are unsatisfactory for a continuous manufacturing line such as the one described above for forming personal products, because the response time required to bring the oven out of bypass mode and into its running mode is lengthy. A major cause of these lengthy response times stems from the relationship between the respective air flow paths used in these prior systems during their running and bypass modes. More particularly, in these prior art systems, a large portion of the ductwork used during the running mode is not used during the bypass mode. Since this unused ductwork has no hot air flow during the bypass mode, it cools down when the oven remains in the bypass mode. Upon restarting of the running mode, this unused ductwork acts as a heat sink for the hot air circulating in these systems and, as a result, these systems may not reach an appropriate operating temperature until the ductwork that was unused during the bypass mode has been warmed up to a satisfactory point.
It is an object of the present invention to provide a convection oven that can be used as part of a substantially continuous manufacturing line.
It is a further object of the present invention to provide a convection oven that can be switched between a running mode and a bypass mode, and which has a fast response time when switched out of a bypass mode and back to a running mode.
It is a still further object of the present invention to provide a convection oven that can be used for heating a substantially continuous supply of nonwoven fibers, which system allows for stoppage of the supply within the oven and which, upon restarting of the supply, outputs fibers that are properly processed.
These and still other objects of the invention will become apparent upon study of the accompanying drawings and description of the invention.